1. Field of the Invention
The present invention relates to a voltage regulator.
2. Description of the Related Art
A conventional voltage regulator will be described with reference to the accompanying drawings.
FIG. 2 is a circuit block diagram showing the structural example of a conventional voltage regulator.
As shown in FIG. 2, a voltage regulator 201 includes external terminals consisting of an input voltage terminal 102, a GND terminal 103, an output voltage terminal 104, and an on/off terminal 110. The voltage regulator 201 also includes a reference voltage circuit 105 that can output a constant voltage, a voltage divider circuit 206 that can divide the voltage of the output voltage terminal 104 at an appropriate ratio, an error amplifier circuit 107 that can adjust an output voltage by comparing two input voltages with each other, an output circuit 108 that can adjust an impedance, a logic circuit 109 that can control the operation of the reference voltage circuit 105 and the error amplifier circuit 107. In FIG. 2, the voltage divider circuit 206 is made up of a resistor 221 and a resistor 222.
Upon inputting an ON signal from the on/off terminal 110, the logic circuit 109 sends a signal to the reference voltage circuit 105 and the error amplifier circuit 107, and makes the output circuit 108 adjust the impedance so that the error amplifier circuit 107 keeps the input voltage from the voltage divider circuit 206 so as to be equal to the input voltage from the reference voltage circuit 105. Therefore, the voltage regulator 201 can keep the output voltage terminal 104 to a constant voltage even if the input voltage fluctuates.
On the other hand, upon inputting an off signal from the on/off terminal 110, the logic circuit 109 sends a signal to the reference voltage circuit 105 and the error amplifier current 107, and adjusts the error amplifier circuit 107 so that the impedance of the output circuit 108 becomes larger. Therefore, the voltage of the output voltage terminal 104 is pulled down to the GND terminal 103 through the impedance of the voltage divider circuit 206, and the voltage regulator 201 can keep the voltage of the GND terminal 103.
The output voltage terminal 104 is connected with various external loads 111 such as a CPU or a microcomputer depending on an intended use. Also, in order to stabilize the voltage of the output voltage terminal 104, the voltage regulator 201 is normally connected with an output capacitor 112 in use.
As described above, in the conventional voltage regulator 201, when the signal is in an off-state, the output voltage terminal 104 is pulled down to the GND terminal 103 through the impedance of the voltage divider circuit 206. Accordingly, in the case where the leak current of the output circuit 108 becomes large due to such conditions that the impedance of the external load 111 becomes large and the temperature of an IC becomes high, the voltage of the output voltage terminal 104 is not pulled down to the voltage of the GND terminal 103. As a result, there arises such a problem that the voltage regulator 201 cannot be turned off.
A simple example in which the leak current of the output circuit 108 becomes large due to such conditions that the impedance of the external load 111 becomes large and the temperature of an IC becomes high will be described.
When the signal is in an off-state, the voltage of the output voltage terminal 104 is represented by the following expression (1).VOUT=ILEAK×(ROUT1//ROUT2)  (1)where VOUT is a voltage (V) of the output voltage terminal 104, ILEAK is a leak current (A) of the output circuit 108, ROUT1 is an impedance (Ω) of the voltage divider circuit 206, ROUT 2 is an impedance (Ω) of the external load 111, and (ROUT1//ROUT2) is a composite impedance (Ω) of the ROUT1 and ROUT2 in parallel.
For example, in the case where ILEAK=1 μA (the value of the maximum presumed leak current), ROUT=3 MegΩ, and ROUT2=∞, the following expression is satisfied from the expression (1).VOUT=1 uA×3 MegΩ=3 V  (2)In this example, in the case where the output voltage of the voltage regulator 201 is 3 V, in both on and off-states, the same voltage is obtained in the above case. That is, the voltage regulator cannot be turned off.
When the voltage regulator 201 cannot be turned off, the external load 111 continues to consume a power wastefully. That is, there arises such a problem that the power consumption of a system using the conventional voltage regulator 201 increases.